1. Field of the Invention
The present invention relates generally to image forming apparatuses employing an electrophotographic method, such as copiers, printers, facsimile machines, or a multifunction peripheral including these functions, and also to developing devices and process cartridges that are provided in such image forming apparatuses. More particularly, the present invention relates to a developing device, a process cartridge, and an image forming apparatus including plural developer conveying units forming circulating paths for the developer.
2. Description of the Related Art
Conventionally, there is known an image forming apparatus such as a copier or a printer provided with a developing device accommodating a two-component developer (in which an additive may be added) including a toner and carriers. Plural developer conveying units are provided in the developing device for performing a developing procedure by circulating the developer in a lengthwise direction (see, for example, patent document 1).
More specifically, the developing device includes a first developer conveying unit (a conveying unit provided with a first screw auger), a second developer conveying unit (a conveying unit provided with a second screw auger), and a third developer conveying unit (a conveying unit provided with a third screw auger). The first developer conveying unit faces a developer carrier (developing magnet roller), and supplies the developer to the developer carrier while conveying the developer in a lengthwise direction (in a direction of a rotation axis of the developer carrier). The second developer conveying unit is disposed at a position that is underneath the first developer conveying unit and that faces the developer carrier, and conveys, in a lengthwise direction, the developer that has separated from the developer carrier. The third developer conveying unit conveys the developer conveyed by the second developer conveying unit to an upstream side of the first developer conveying unit, and conveys the developer that has reached the downstream side of the first developer conveying unit to an upstream side of the first developer conveying unit.
That is, the first developer conveying unit has a function of supplying the developer to the developer carrier, the second developer conveying unit has a function of collecting the developer from the developer carrier, and the third developer conveying unit has a function of forming a circulating path to the upstream side of the first developer conveying unit.
In such a developing device, circulating paths are formed in the lengthwise direction, and therefore the developer does not become uneven along the lengthwise direction. Accordingly, the developer can be efficiently and sufficiently mixed and stirred, and the size of the device in the widthwise direction (a direction orthogonal to the lengthwise direction) can be relatively small. Particularly, the developer in the device can flow smoothly by providing the first developer conveying unit for supplying the developer and the second developer conveying unit for collecting the developer, which are facing the developer carrier.
Patent Document 1: Japanese Laid-Open Patent Application No. 2001-249545
The problem with the developing device disclosed in patent document 1 is that the toner scatters outside the developing device and soils the inside of the image forming apparatus, and the scattered toner soils the output image. Particularly, a significant amount of toner scatters from the second developer conveying unit for collecting the developer that has separated from the developer carrier.
This is because the plural developer conveying units are partitioned by walls, and the internal pressure changes.
Specifically, in the second developer conveying unit, a suction airflow is generated as the developer carrier rotates, and the internal pressure becomes higher than the external pressure (positive pressure). Accordingly, the toner floating inside the second developer conveying unit is blown out from gaps of the device (for example, from gaps at both edges of the developer carrier in the lengthwise direction) and scatters outside.
Furthermore, in the first developer conveying unit, a blowout airflow is generated as the developer carrier rotates, and the internal pressure becomes lower than the external pressure (negative pressure). Accordingly, the efficiency of passing the developer from the first developer conveying unit to other developer conveying unit decreases, and redundant toner scatters outside the device.
The above phenomena are particularly significant in a high-speed machine in which the developer carrier and conveying members provided in the developer conveying units revolve at high rotational speeds.